A "safe" mip-level is one that has exactly half the dimensions of
the current level. In other words, we guarantee that we can halve the
dimensions 4 times without loss (i.e. odd dimension).
This is achieved by effectively padding the viewport (i.e. making the
viewport's dimensions multiple of 16). The post-processing passes
eventually take care of cropping the extra padding so the final
image is unchanged.
This is a more generic solution to dynamic resolution padding, helps
with memory allocations and helps all the algorithms that depend on
mipmaping (e.g. DoF, SSAO, Bloom).
One important improvement brought by this change is that all "final"
post-processing effects are now able to handle a sub-region of the
source, including: color-gradding, fxaa and upscaling.
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